LED arrays are distinguished by high efficiency, long life, fast response times and relatively low sensitivity to impacts and vibrations. For this reason, LED arrays are being used with increasing frequency in lighting devices where incandescent lamps have often been used heretofore, particularly in motor vehicle headlights, reading lamps or flashlights. Because of their high luminosity, LED arrays have also now proven suitable for use in traffic signal systems or projected light sources.
In LED arrays that are used for such lighting purposes, the LED chips are usually driven at high operating currents to obtain the highest possible luminance. For example, operating currents of 1 A or more may occur. So that the electrical contacting of the semiconductor chip is able to withstand such high currents over the long term, bonding wires 30 μm or more in diameter are typically used to contact semiconductor chips in high-power LED modules. However, the use of these relatively thick bonding wires causes some of the radiation emitted by the LEDs to be blocked by the bonding wires. In addition, the bonding wires make it difficult to mount optical elements close above the surface of the semiconductor chip, since the radius of curvature of the wires, and thus the amount of space they occupy above the chip surface, cannot be reduced arbitrarily.
In applications of LED arrays, particularly in projected light sources, it is necessary for light to be emitted within the tightest possible range of solid angles. Since LEDs, however, usually have a relatively broad range of emission angles, optical elements are necessary for this purpose. It is desirable in this case to get the optical elements as close as possible to the semiconductor chips, in order to achieve compact construction and to be able to make use of the largest possible fraction of the radiation emitted by the semiconductor chip. This is hampered, however, by the use of bonding wires for contacting.
From the document WO 98/12757, it is known to embed an optoelectronic semiconductor chip, contacted with a bonding wire in a conventional manner, in a potting compound containing luminescence conversion materials, in order to convert at least a portion of the radiation emitted by the semiconductor chip to higher wavelengths. In this way, a semiconductor chip that emits blue or ultraviolet light, for example, can be used to generate mixed-color or white light. However, embedding the semiconductor chip in a potting compound raises problems in the case of LED arrays, which are operated at relatively high currents, since the potting compound impedes heat dissipation from the semiconductor chip.
The document WO 97/50132 also describes applying a luminescence conversion layer to a cover plate of an LED housing, an LED chip contacted with a bonding wire in a conventional manner being positioned in the housing.
Described in the document DE 19603444 is an LED device in which the LEDs are contacted by means of metallic conductive traces that are applied to a transparent plate. In this type of contacting, a portion of the radiation emitted by the LEDs is absorbed in the metallic conductive traces.